Generally, wood screws defining a peripheral spiral is used, rather than ordinary nails, to join wooden articles rigidly, for example. This is because the wood screws have a greater tightening force and a shorter curing period, with the peripheral spiral cutting into the wood fiber to perform a retaining function.
However, an operation to screw in the wood screw requires the wood screw to be maintained constantly upright while being turned, to be pressed hard in a screwing direction, and to maintain two boards to be joined in pressure contact with each other. Expert skill is essential to this screwing operation. If, for example, the wood screw tilts during the screwing operation, the spiral intended to perform the retaining function will damage the wood fiber, thereby reducing the tightening force, and greatly deteriorating joinder retention.
Moreover, when the wood screw is extended through a mounting board and subsequently screwed to a board to be attached, the screwing speed is reduced, a gap tends to be formed between the two boards, and the operation is time-consuming.
When wooden articles are joined by nails or wood screws, the wooden articles are just penetrated by the nails or wood screws. No force is applied for positively maintaining the two boards in pressure contact. Gaps tend to occur when the joinder retention deteriorates with vibrations or the like of products during transport. Further, retaining strength is variable with types of surfaces to which wood screws are applied. The retaining strength is weaker for a butt end surface than for a cross grain or straight grain surface.
When articles are joined by wood screws, the effect of design is impaired by the large, slotted or Phillips heads of the wood screws. A covering operation to avoid the reduction in the design effect is not easy or effective since the heads of the wood screws are relatively large.